Carbon nanotubes are tubular structures formed by one or more layers of graphene. Since the discovery of a synthetic process to make carbon nanotubes in the early nineties, lot of attention has been concentrated on CNTs because of their excellent electrical, thermal and mechanical properties and large specific surface. Based thereon, all kinds of applications have been suggested, ranging from microelectronic components, displays, radio communication to fuel cells.
There are three main approaches for the synthesis of single- and multi-walled carbon nanotubes, including electric arc discharge of graphite rod, laser ablation of carbon, and chemical vapor deposition of hydrocarbons. However, the most cost-effective methods for synthesizing carbon nanotubes have been based on chemical vapor deposition (CVD). Metal catalyzed thermal CVD typically uses a cheap feedstock and has relatively low energy requirements, and has therefore attracted interest for the purpose of bulk synthesis. In CVD methods, a carbon-containing gas is decomposed at high temperatures and under the influence of a finely divided catalyst (usually iron, nickel, cobalt or other transition metals or alloys) the carbon atoms are bonded to form CNTs. Catalyst particles may be manufactured in situ by the decomposition of metalloorganic compounds or may be inserted into the CVD furnace on a fixed substrate.
EP 1952467 is directed to nanowire structures and interconnected, porous nanowire networks comprising such structures. The nanowire serves as a core for a template for the growth of carbon networks. To maximize catalyst accessibility and utilization, for instance in fuel cell applications, EP'467 teaches to deposit a thin film or layer of metal catalyst onto the surface of the nanowires. However, the intimate linkages between nanowire support and the catalyst particles deposited thereon reflects on the limited catalyst efficacy and conductivity properties. Additionally, the catalyst particles located on top of the carbon structure makes those more vulnerable to desorption. Also, the nanostructures thus obtained are non-crystalline, which would render these structures less suited for many applications. The contents of EP'467 is herein incorporated by reference.
It is widely recognized in the art that CVD typically results in a large amount of impurities due to little control over catalyst properties besides other reasons. US 2006/0104889 teaches catalysts of small average particle sizes with narrow size distribution, otherwise difficult to synthesize. US'889 proposes catalyst particles having a size between 1 to 50 nm supported on a powdered oxide at a 1:1-1:50 particles to support weight ratio. The contents of US'889 is herein incorporated by reference.
EP 1334064 discloses a process for preparing carbon nanotubes, which comprises suspending nanometer-sized fine metal particles in a gaseous phase. It enables control of the shape and structure of the carbon nanotubes. The metal nanoparticles have an average size in the order of a few to several hundred nanometers. It is attempted to control CNT purity through the use of surfactant, which should prevent cohesion of the colloidal metal particles. The contents of EP'064 is herein incorporated by reference.
EP 2123602 discloses nanotubes grown using CVD process in which an S-layer of proteins is generated on a substrate and used as a mask where inorganic nanoparticles are deposited on through the incubation and and reduction of the corresponding metal salt solution. It suggests a physical assembly of discrete nanostructures in FIGS. 1a, 1b and 2, without any chemical interconnectivity between those structures. The contents of EP 2123602 is herein incorporated by reference.
However, the narrow size distributions of metal catalyst particles applied in the art such as the above can only be provided in low densities. Even if use is made of a micro-emulsion to stabilize the metal particles, metal particle concentrations are typically of about few mM, on the penalty of break-up. For that particular example in EP 1334064, the maximum concentration of metal particles is 10 mM. Hitherto, at these kinds of concentrations however networks of carbon nanostructures have not been observed in the art.
Also, regardless of the above attempts to control particle size in CVD, Takenaka et al. “Formation of carbon nanotubes through ethylene decomposition over supported Pt catalysts and silica-coated Pt catalysts” Carbon 47 (2009) 1251-1257, shows that an initially controlled size of metal particles is no warrant for success, because without any preventative measures the metal particles seriously aggregate during the actual carbon decomposition.
Hence, the art strives for better control of catalyst aggregation, and consequently, the purity and uniformity of the carbon nanostructures. There is also a need for simple CVD-based methods to produce chemically interconnected carbon nanostructure networks.